Aggregatibacter actinomycetemcomitans (Aa) is a key pathogen in the polymicrobial infection of periodontitis. Colonization by distinct clonal lineages of Aa may lead to different consequences, from minimal diseases to the development of aggressive periodontitis. The basis for the variable virulence of Aa is not fully understood, an presents a challenge to effective diagnosis and management of Aa-associated periodontitis. Aa demonstrates significant variation in gene content primarily because of genomic islands, which have been known to be associated with bacterial virulence potential. The long-term goal of our research plan is to identify the full spectrum of the virulence determinants of Aa. Toward that goal, the objective of this study is to define the role of genomic islands in the pathogenesis of A in periodontitis. The central hypothesis is that the virulence of Aa is modulated by genomic islands and mediated by Th17 cells and counteracted by Treg. The hypothesis is formulated based on the preliminary studies of the 171 Aa genomic islands that demonstrated (i) disease-association of specific island genes, (ii) distribution of some island genes limited to Aa strains f high virulence, (iii) homology of individual island genes to known virulence factors of other pathogens, and (iv) patterns of expression of the island genes that suggested functionality. Two aims will be pursued: Aim 1. In vitro functional analysis of genomic islands of Aa. The working hypothesis is that genomic islands are expressed in in vivo like conditions and modulate the virulence expression of Aa. Aim 2. In vivo characterization of the role of genomic islands to Aa virulence. The working hypothesis is that the genomic islands modulate the virulence of Aa in periodontitis via Th17 and regulatory T cell immune responses; the former leads to osteolysis and tissue destruction, while the latter dampens the inflammatory response. The hypothesis will be tested with a novel animal model of rats. The outlined studies will for the first time evaluate and identify virulence-associated genomic islands of Aa and reveal their pathogenic mechanisms. The results are expected to have an important positive impact, because the information will expand the knowledge to the pathogenesis of Aa leading to improved diagnostics and treatment of periodontal infections.